Reducing protein can lengthen life, improve health, studies in mice and people suggest

To live longer, lower your protein intake. In two independent studies, people and mice eating diets low in protein were healthier and tended to live longer than those eating protein-rich diets. Both studies, which appear in the March 4 Cell Metabolism, suggest that animal proteins, including those from meat and dairy, are less healthy than plant proteins.

In a study of 6,381 people 50 and older, those age 65 and younger who got less than 10 percent of their calories from protein had lower risks of dying from cancer and diabetes during 18 years of follow-up than those who ate more protein. People who ate moderate amounts of protein — making up 10 to 19 percent of the diet — had, for instance, three times the chance of dying from cancer as those on a low-protein diet. After age 65, though, the pattern reversed with high-protein diets (20 percent or more) carrying lower risks of dying of cancer.

“A high-protein diet is one of the worst things you can do up to age 65,” says Valter Longo, a biogeriatrician at the University of Southern California in Los Angeles who coauthored the study. Eating lots of protein, especially protein from animal sources, can be nearly as harmful to health as smoking, he says.

Other researchers aren’t ready to vilify proteins. The findings, says Edward Giovannucci, an epidemiologist at the Harvard School of Public Health, “are sort of intriguing, but I would view them as not really definitive at this point.”

He finds it particularly puzzling that low-protein diets are healthy before age 65 but become harmful in old age. More studies are needed to confirm the results, he says.

In the other new study, protein proved more important than calories for determining health and longevity. Researchers fed 858 mice fed one of 25 diets that had varying ratios of proteins, carbohydrates, fats and calories, Those that ate low-protein, high-carbohydrate diets fared the best. As protein content climbed, the mice’s risk of dying younger also increased.

In contrast, dropping calories without lowering protein generally did not affect the rodents’ life spans. That result seems to contradict previous studies that have shown that cutting calories lengthens life span in organisms including yeast, fruit flies, dogs and mice. But coauthor David Le Couteur, a biogerontologist at the University of Sydney, says the study may shed light on how caloric restriction actually extends life.

“We think that calorie restriction works not by restricting the amount of energy, but by restricting protein,” he says.

Previously, researchers have mimicked the health effects of a low-calorie diet in mice with a drug called rapamycin. Rapamycin inhibits the action of a protein called mTOR. That inhibition turns on a variety of biological processes that help slow aging, scientists have discovered.

In the new study, mice eating low-protein, carb-rich diets also had lower mTOR activity in their livers than mice on protein-rich diets did. Particularly important for controlling mTOR activity were the blood levels of certain amino acids that are more common in animal proteins than in plant proteins. Lower levels of those amino acids corresponded to lower mTOR activity, better health and longer lives for the mice.

The study’s one exception seemed to be mice that had low-protein, high-fat diets. Those animals were less healthy and died younger.

The results confirm findings with fruit flies that showed reducing protein consumption can extend life, says Subhash Katewa, a researcher who studies aging at the Buck Institute for Research on Aging in Novato, Calif. But people are more complex. How changing protein levels affect a person’s health will depend upon many additional factors, he says. “If your body can sustain protein restriction, adults should try it,” Katewa suggests.

There’s no downside to eating less protein or to replacing meat with plant proteins such as those found in beans and nuts, says Longo. “If we’re wrong there’s no negative side effect. If we’re right it means a reduction in cancer and diabetes.”

Editor's Note: This story was updated on March 18, 2014, to correct that the risks reported were of dying from cancer and diabetes, not of contracting the diseases.